1. Field of the Invention
The present invention relates to a tunable laser apparatus and a photoacoustic apparatus that uses this laser apparatus.
2. Description of the Related Art
Photoacoustic tomography (PAT) apparatuses that use lasers for medical applications have been developed (see Non-Patent Literature 1). PAT apparatuses are expected to be used for determination of presence or absence of a tumor through observation of blood vessels congregating around a tumor in a living body, or for functional analysis of tissues based on the differences between oxyhemoglobin and deoxyhemoglobin spectra.
A PAT apparatus emits nanosecond laser pulses to a segment being measured and analyses received signals converted from ultrasound waves (photoacoustic waves) generated by the photoacoustic effect to obtain an image. As the light intensity decays by diffusion in the living body, it is necessary to emit laser beams with high energy per pulse to obtain a photoacoustic wave from a segment relatively deep in a living body such as a breast, in particular.
Some lasers called tunable lasers that use titanium sapphire or alexandrite, which is a laser medium with a broad gain bandwidth, allow selection of a wavelength. Generally, to induce laser oscillation at a desired wavelength, tunable lasers use a wavelength selection mechanism that includes an optical element such as a prism, a diffraction grating, or a birefringent plate. The optical element is arranged on an optical axis inside a resonator and mechanically moved to allow selection of a wavelength at which oscillation should take place.
If a prism is used as an element of the wavelength selection mechanism, generated beams of respective wavelengths are resonated in the resonator correspondingly to the angular deviation of the optical paths that is determined by the refractive index of the prism. If the prism itself is used as a reflective mirror of the resonator, the prism is mechanically rotated in the plane of the substrate forming the resonator. If a separate reflective mirror is used with the prism, the prism is fixed in position, while the reflective mirror located correspondingly to the angular deviation of the optical path of respective wavelengths is mechanically rotated.
If a diffraction grating is used as an element of the wavelength selection mechanism, a reflective mirror is mechanically rotated in accordance with an oscillating wavelength for selection of the target wavelength, as with the prism.
If a birefringent plate is used as an element of the wavelength selection mechanism, the birefringent plate is mechanically rotated such as to maintain the angle of the optical axis of the beam being resonated relative to the incident and exit surfaces of the birefringent plate at a constant angle.
In another known method (Patent Literature 1), light of two different wavelengths is produced at the same time in the resonator, and with a wavelength selective filter disposed outside the resonator inserted onto the optical axis or retracted from the optical axis, a target wavelength is selected.    Patent Literature 1: Japanese Patent Application Laid-open No. 2007-248304    Non-Patent Literature 1: S. Manohar et al, Proc. of SPIE vol. 6437 643702-1